Instrument and method for testing local area network cables

ABSTRACT

An instrument and method for the troubleshooting and verification of copper-wire local area network (LAN) cable systems provides a series of resistance and capacitance measurements between all possible wire pairs using a pair of switch matrices and a software method for evaluating the measurements which are stored in a two-dimensional matrix. The stored measurements are evaluated against predetermined figures of merit and the decision pattern in matrix form is compared to that of a known good LAN cable system. Error messages responsive to mismatches are generated and presented to the operator via a user interface. Near-end cross talk (NEXT) isolation is evaluated using a mathematically calculated differential capacitance technique. Error messages are generated responsive to difference capacitance values larger than a figure of merit correlated to an acceptable level of NEXT isolation.

BACKGROUND OF THE INVENTION

This invention relates generally to devices employed in the testing oflocal area network (LAN) cables and in particular to an instrument whichtests the relevant parameters of a copper-conductor LAN cable bymeasuring the capacitance between all pairs of conductors and employinga software method operating on the stored measurement data.

Local area networks (LAN's) now connect a vast number of personalcomputers, workstations, printers, and file servers in the modem office.A LAN system is most commonly implemented by physically connecting allof these devices with copper-conductor twisted-pair LAN cables, the mostcommon being an 8-wire cable which is configured in 4 twisted-wire pairswith each end of the cable terminated in an industry-standard connector.Some LAN cables include a flexible foil wrapper that acts as anelectrostatic shield. In a typical installation, LAN cables may berouted through walls, floors, and ceilings of the building. LAN cablesystems require constant maintenance, upgrades, and troubleshootingbecause LAN cables and connectors are subject to breakage, offices andequipment must be moved, and new equipment are added.

The tasks of installing, replacing, or re-routing cables typically fallon a professional cable installer or in-house network maintenanceperson. During the installation phase, each cable is routed through thebuilding and a connector is attached to the each end of the new cable.Each wire in the cable must be connected to its proper respectiveelectrical connection at both ends of the LAN cable in order for the LANconnection to function properly. A variety of LAN cables are used in theindustry, including: unshielded twisted pair ("UTP"), shielded twistedpair ("STP"), and coaxial cables. LAN cable installation practices,cable performance specifications, and building wiring practices aregoverned by the Electronic Industries Association Commercial BuildingTelecommunications Wiring Standard EIA/TIA 568.

Such connections can be tested with an electrical resistance-measuringinstrument commonly known as an ohmmeter which tests the direct current(d.c.) resistance through the electrical path between the ohmmeter'stest leads. Using the ohmmeter to effectively test a LAN cable requiresdetailed knowledge of the proper connections. The end of the LAN cablesystem in which the test instrument is applied is the "near-end". Theother end of the LAN cable thereby becomes the "far-end". With a knowntermination such as resistors at the far-end of the cable to provide acomplete circuit, the "wire map" or set of connections can be discerned,along with short-circuit and open-circuit wiring errors. This manualtechnique of probing connections quickly becomes prohibitivelyinefficient and time-consuming. For a cable of N wires, a total ofN!/(N-2)! measurements must be performed for a complete test between allpairs of the N wires, probing each respective pair with both negativeand positive polarity.

Specialized LAN cable test instruments have been developed to diagnosethe most commonly encountered cable problems. The instrumentautomatically performs a series of resistance measurements therebyrelieving the operator of the burden of probing individual connectionsmanually. The instrument performs continuity checks on the cable toensure that all the connections exist as required by industry standarddefinitions and provides the operator with a visual indication ofcontinuity and proper connection of each wire pair through the cable.Tests for open-circuit errors, short-circuit errors, and crossed pairerrors are provided.

The least expensive LAN cable test instruments are essentiallyspecialized ohmmeters equipped to test industry-standard terminationsand wire maps according to EIA/TIA-568. Such instruments allow fordirect connection to the LAN cable, along with a special terminatingdevice ("cable identifier"), which is connected to the far end of thecable to facilitate a known return path for the d.c. test currentprovided by the instrument for each specified wire pair throughresistors with pre-determined values and diodes with pre-determinedpolarities. Such instruments suffer from several marked disadvantageshowever. First, because the instrument is limited to d.c. measurements,the set of tests that the instrument can perform is limited, leavingmany of the potential faults in a cable undetected. A critical parameteris the amount of coupling between wire pairs, commonly referred to ascross-talk. Each of the four wire pairs is twisted together within thecable and that respective pairing must be maintained in order to getproper isolation from the other pairs. A common wiring error is to crossa wire pair at one end of the cable while duplicating the error at theother end. While the connection appears proper according to the d.c.measurement, the wire pairs are no longer twisted separately but are nowcommingled, resulting in an unacceptable level of cross-talk between thetwo wire pairs.

The second disadvantage of cable mapping instruments is that a cableidentifier must be connected to the far-end of the cable being tested toprovide a known return path for the d.c. test current. When the far-endof the cable is hundreds of feet away, the process of testing the cablebecomes cumbersome, often requiring two people, one at the near-end andone at the far-end, to perform the task of troubleshooting and verifyingmultiple cables, often long after the initial installation has beencompleted, resulting in costly re-work.

More sophisticated LAN cable test instruments are often equipped forevaluating cross-talk between wire pairs at the near-end of cablethrough standardized near-end cross talk (commonly referred to as"NEXT") measurements. NEXT is a measure of the level of isolationbetween separate wire pairs. A NEXT test is typically performed byinjecting a high frequency alternating current (a.c.) test signal into awire pair at the near-end of the cable, often at frequencies similar toactual data rates which range as high as 16 MHz, measuring the signallevel induced in each of the other pairs as measured at the near-end,and comparing the induced signal level with the injected signal level todetermine the level of isolation. A higher level of isolation betweenwire pairs is necessary to avoid interference between data communicationpaths. The consequence of inadequate isolation between wire pairs isdegraded communications reliability and increased error rates.Inadequate NEXT isolation is a symptom of a number of possible problemsincluding incorrectly wired LAN connectors or telephone-grade cablesthat do not meet the specifications for data communications. Thedisadvantage of an instrument providing only a NEXT reading is that theinstrument may not provide any additional information regarding thesource of the problem, leaving the operator to troubleshoot the problem.

Accordingly, an instrument that can provide troubleshooting andverification of the proper connection of a variety LAN cable systemswith a more complete set of automated diagnostic tests without thenecessity of applying a cable identifier at the far-end of the LAN cablewould be desirable.

SUMMARY OF THE INVENTION

In accordance with the present invention, an instrument fortroubleshooting and verifying copper-wire LAN cables by measuring theresistance and capacitance between all possible combinations of wiresand employing a software method of analyzing the measurements isprovided. The LAN cable system to be tested consists of the LAN cableplus the connectors on either end. The near-end of the LAN cable systemis coupled to one of several industry-standard connectors on theinstrument. A series of resistance measurements is made between allpossible combinations of wire pairs in the cable which are selectedusing switch matrices coupled to a resistance measurement unit. Theresistance measurement results are stored in digital memory in the formof a resistance matrix.

The stored resistance matrix values are then evaluated using theinstrument microprocessor to detect the presence of a remote cableidentifier on the other end of the cable as well as any shortedconnections. A remote cable identifier, designed to augment theresistance measurement functions of the instrument when conditionsallow, contains a system of diodes and resistors with predeterminedvalues corresponding to a unique cable identifier number. With the cableidentifier coupled to the far-end of the LAN cable system, each wirepair may be tested for polarity reversal and far-end open-circuiterrors, and the respective cable identifier number is determined for thepurpose of identifying a particular LAN cable system. With no cableidentifier attached to the far-end of the LAN cable system, theresistance measurement is used to check for short-circuit errors.Short-circuit errors are tested for by comparing the measured resistancevalues to a figure of merit for short-circuit errors based on apredetermined threshold value. Short-circuit errors result in measuredresistance values less than the figure of merit and, once ashort-circuit error has been detected, distance of the short-circuiterror from the near-end of the cable can be calculated by dividing themeasured resistance by the resistance per unit of distance expected forthat type of cable. Decisions relating to short-circuit errors,open-circuit errors, and diode polarities are stored symbolically inmemory in a matrix format for further comparison to predeterminedpatterns of the expected LAN cable system to detect crossed-pair andpolarity reversal errors. Mismatches detected between the derivedpattern of the symbolic matrix and the expected pattern will generate anerror signal to the operator.

The instrument next performs a series of capacitance measurementsbetween all possible combinations of wire pairs which are selected usingswitch matrices coupled to the same measurement unit which is nowconfigured for capacitance measurements. The measurement data are storedin digital memory in the form of a capacitance matrix. The storedcapacitance matrix values are then evaluated to determine which wirepairs are twisted together, based on the phenomenon that wire pairstwisted together have a significantly higher capacitance value than thatof adjacent wires not twisted together. Furthermore, open-circuit errorsat the near-end of the cable are tested for by comparing the measuredcapacitance values to a figure of merit for open circuits based on apredetermined fraction of the maximum measured capacitance value.Open-circuit errors result in measured capacitance values significantlyless than other measured capacitance values. The decisions regardingopen-circuit errors and wire pairing are stored symbolically in memoryin a matrix format for further comparison to predetermined patterns ofthe expected LAN cable system to detect split-pair errors. Mismatchesdetected will generate an error signal to the operator.

Finally, a difference capacitance matrix is calculated based on theassigned wire pairs. The difference in capacitance between each wire ofthe pair to all other wires in the cable is calculated in order todetermine a relative figure of merit for near-end cross talk (NEXT).NEXT is a measure of the relative voltage isolation between wire pairsin which a signal in one wire pair induces a voltage in another wirepair via capacitive coupling. If the capacitance values between each ofthe wires of the wire pair to another wire is perfectly balanced, thecapacitance values will match and the respective entry in thecapacitance difference matrix is zero. A low difference capacitanceprecisely correlates to a high degree of voltage isolation between thewire pair and the other wires because the differential voltages inducedthrough capacitive coupling between the wire pair and the other wireswill be zero. Conversely, a large difference capacitance value signifiesan unacceptably low level of NEXT isolation. Possible causes of low NEXTisolation include split-pair wiring errors and open-circuit errors inthe wire pair being tested that result in a difference in the electricallength of the wires.

Unlike a conventional NEXT measurement which directly measures theisolation between wire pairs by comparing analog signal levels at apredetermined high frequency, the instrument according to the presentinvention directly utilizes the capacitance measurements stored in thecapacitance matrix to calculate a difference capacitance for eachpossible wire pair which are stored in a difference capacitance matrix.A predetermined figure of merit based on empirical measurements thatcorrelate to a threshold level of acceptable NEXT isolation is used tocompare against the values in the difference capacitance matrix. Anyvalue in the difference capacitance matrix higher than the figure ofmerit is a fault which will generate an error signal.

One feature of the present invention is to provide a method andapparatus for troubleshooting and verification of copper-wire local areanetwork (LAN) cable systems using a series of resistance and capacitancemeasurements and software for evaluating the measurements.

Another feature of the present invention is to provide an instrument fortroubleshooting and verifying copper-wire LAN cable systems at thenear-end with no need to connect a cable identifier to the far-end ofthe cable.

A further feature of the present invention is to provide a test for thenear-end cross talk (NEXT) parameter using a technique of mathematicallycalculated difference capacitance.

An additional object of the present invention is to provide aninstrument for troubleshooting and verifying copper-wire LAN cableswhich is readily adaptable to a wide variety of LAN cable types.

Other features, attainments, and advantages will become apparent tothose skilled in the art upon a reading of the following descriptionwhen taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are respectively is an illustration of a physicalcopper-conductor LAN cable and LAN cable connector;

FIG. 2 is a circuit schematic of a LAN cable system consisting of acopper-conductor LAN cable and LAN cable connectors on either endshowing the expected pairing of the wires;

FIG. 3 is a circuit schematic of a LAN cable illustrating a split-pairwiring error and a crossed-pair wiring error;

FIG. 4 is a circuit schematic of a LAN cable illustrating anopen-circuit wiring error, a short-circuit wiring error, a polarityreversal wiring error, and an open-shield wiring error;

FIG. 5 is a block diagram of a LAN cable test instrument employing amethod of resistance and capacitance measurement of all possible wirepairs;

FIG. 6A-6C form a flow chart diagram illustrating the program executedby the microcontroller of the present invention;

FIG. 7 is a diagram illustrating the matrix data structure format inwhich the resistance data are stored;

FIG. 8 is a diagram illustrating the treatment of the resistancemeasurement data into decision related information stored in a symbolicformat;

FIG. 9 is a diagram illustrating the matrix data structure format inwhich the capacitance data are stored;

FIG. 10 is a diagram illustrating the treatment of the capacitancemeasurement data into decision related information stored in a symbolicformat;

FIG. 11 is a diagram illustrating the matrix format showing how thecapacitance difference matrix is calculated; and

FIG. 12 is an illustration of the physical end view of an 8-wire LANcable which illustrates the relationship of the physical capacitanceelements between the wires calculated in the capacitance differencematrix.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1A and FIG. 1B, there is shown generally a LAN cable10 which contains insulated copper-conductor wires 40 which commonlynumber between four and eight. The insulation on the wires iscolor-coded in order to facilitate proper connections of the cable to aconnector 30. The wires are twisted together into wire pairs within aLAN cable to order to maximize the electrical isolation of each pairfrom the other pairs. The twist rate and other electrical and mechanicalparameters are specified and well known in the industry for a data gradeLAN cable. A LAN cable may be shielded, meaning that a shield 20surrounds the wires of the cable as a conductive sheath to reducesusceptibility to external interference and also to reduce theelectromagnetic emissions from the cable induced by the datatransmission. The connector 30 is typically an eight-conductortelephone-type connector commonly referred to in the industry as anRJ-45 connector.

Referring now to FIG. 2, the schematic diagram illustrates a completeand error free LAN cable system consisting of the LAN cable 10 andconnectors 30 on either end. The wires 40 are connected as shown inconformance with EIA/TIA-568. A twisted wire pair consists of twoindividual wires that are twisted together along the length of the cableto achieve electromagnetic isolation from other twisted wire pairs inthe cable and from external sources of interference. A LAN cable 10contains eight wires 40 typically formed as four twisted wire pairs. Theshield 20, if present, is connected to the SHIELD connection of theconnector 30 on either end of the LAN cable 10. When testing the LANcable system, the end of the LAN cable 10 in which the LAN testinstrument is connected is referred to as the "near-end" and the otherend of the LAN cable 10 is the "far-end".

FIG. 3 illustrates examples of several common wiring errors in a LANcable system. Reversing the wires connected to connections 2 and 3 ofthe connector 30 on both ends of the cable creates a split-pair wiringerror 50a which results in unacceptably poor isolation between the twodata paths. Transposing two entire data paths, such as connections 4 and5 for connections 7 and 8 on one end of the LAN cable 10 results in acrossed-pair wiring error 50b.

FIG. 4 further illustrates several more common wiring errors in a LANcable system. An open circuit is a current path having a very high d.c.resistance, a condition normally expected between any pair of wires 40of a LAN cable system with no terminations that provide current paths atthe far-end. An open-circuit wiring error 50c can occur anywhere in theLAN cable system but is most commonly the result of failure of a wire 40to make electrical contact at the connector 30. A short circuit is acurrent path having a relatively low d.c. resistance. A short-circuitwiring error 50d can occur between any two wires in the LAN cable systemand may result from a defect in the wire insulation or as the result ofthe LAN cable 10 being physically crushed at some point along itslength. Reversing the wires of a wire pair at one end of the LAN cable10 results in a polarity-reversal error 50e. If the shield 20 ispresent, a failure to make electrical contact between the shield 20 andthe SHIELD contact of connector 30 is an open-shield error 50f.

Referring now to FIG. 5, there is shown the block diagram of acommercial embodiment of a LAN cable test instrument 100. An inputconnector 110 accepts the connector 30 which may connect to any of avariety of industry-standard connector types, including DB-9, RJ-45, andBNC in the commercial embodiment. Each contact of a input connector 110is coupled via a system of parallel input lines 120 which are numbered 1through 8 to an input of a switch matrix 130 and a corresponding inputof a switch matrix 140. The shield 20 of LAN cable 10 is coupled to ahousing of the connector 110. Input connector 110 accepts up to eightinput lines in the commercial embodiment but the architecture is readilyextendible to include more lines. An output of the analog multiplexer130 is coupled to an input of a measurement unit 150 and an output ofthe switch matrix 140 is coupled to a second input of the measurementunit 150. The housing of connector 110 is coupled to an input labeledShield of measurement unit 150. Switch matrices 130 and 140 provide themeasurement unit 150 with switching capability for the purpose ofcoupling the respective inputs of the measurement unit collectivelylabeled Input to any two of the input lines 120 for measuring electricalparameters at said Input which include the resistance through theselected wire pair and capacitance across the selected wire pair. Themeasurement data is coupled from the output of the measurement unit 150to an input channel of the microcontroller 160.

The microcontroller 160 performs the functions of measurement control,measurement processing, and user interaction. In the commercialembodiment, microcontroller 160 is a microprocessor containing its ownmemory including RAM (random access memory) and ROM (read-only memory)in a single integrated circuit. The RAM is used for storing measurementdata and test results while the ROM contains the system controlinstructions which are the software program executed by microcontroller160 for controlling the overall operation of the instrument 100. It willbe obvious that separate microprocessor, RAM, and ROM integratedcircuits may also be effectively employed in place of themicrocontroller 160. Microcontroller 160 is coupled to the measurementunit 150 and to the switch matrices 130 and 140 via the measurementcontrol bus 190 to control the selection of wire pairs to be tested,along with the operation of the measurement unit 150. During the processof taking a measurement, microcontroller 160 controls the measurementunit 150 by selecting the measurement function, either resistance orcapacitance, as well as providing a trigger signal to make themeasurement and return the measurement data. Simultaneously,microcontroller 160 determines which of the input fines 120 are coupledto the measurement unit 150 by sending commands via measurement controlbus 190 to the switch matrices 130 and 140. During the measurementprocess, the measurement values provided by the measurement unit 150 arestored in random access memory (RAM) in the microcontroller 160 in theconventional manner. For example, during a capacitance measurementbetween lines 1 and 2 of the input lines 120, the microcontroller 160couples the input of the measurement unit 150 to lines 1 and 2 bysending the appropriate digital control signal to the analogmultiplexers 130 and 140, sets the measurement unit 150 to capacitancemode, and obtains a capacitance measurement value which is storedsymbolically in RAM as C12. Separate memory locations are maintained inRAM for storing capacitance and resistance measurement data.

The microcontroller 160 is coupled to peripheral devices to communicatewith the user, displaying the instrument status and test results for theuser on a display 170 and responding to user input via a user controlinterface 180. In the commercial embodiment, display 170 is a liquidcrystal display (LCD) and the user control interface 180 is a keypad androtary switch. The addition of other peripheral devices such as audiotone generators, external displays, and keyboards will be obvious.

As an overview, FIG. 6A, 6B, and 6C together illustrate the process bywhich the measurements are taken and the data is processed in order toreach conclusions about whether the LAN cable system being tested isfree of fault. The process is logically divided into three majorsections. First, as shown in FIG. 6A, the resistance between allpossible wire pairs is measured and evaluated for short-circuit wiringerrors 50d. If a cable identifier is present, more detailed evaluationsare performed. Second, as shown in FIG. 6B, the capacitance between allpossible wire pairs is measured and evaluated for open-circuit wiringerrors 50c, wire pairs are determined, and the pairings are evaluatedagainst figures of merit. Third, as shown in FIG. 6C, the cross-talkisolation between all identified pairs of wires is evaluated using acalculated matrix of values based on the measured capacitance values.

Referring now to FIG. 6A, there is shown a flow chart illustrating theprocess by which the resistance measurements are performed. The testsequence begins with block 200. The test may be initiated by usercontrol or on a continual basis with one test automatically followinganother.

The microcontroller 160 moves on to block 210 to read the type of LANcable to be tested, such as unshielded twisted pair (UTP), shieldedtwisted pair (STP) or coax, the wire type, and cable configuration. Anaffirmative request of the user via control interface 180 such asthrough the selection of a menu displayed on display 170 can be used tochange the type of LAN cable. This setting of LAN cable type primarilyaffects the type of tests that will be performed. The specification thatfollows is for UTP, the most commonly encountered cable type. If STP isselected, the instrument 100 will also check for an open-shield error inaddition to the measurements performed for UTP cable. If coax isselected, only one wire and the shield will be tested using a limitedsubset of measurements.

The measurement of resistance values between all pairs of wires isconducted in measurement block 220. Referring now to FIG. 7, the datastructure in which the resistance measurement data are stored in digitalmemory is a resistance matrix 500. A total of 56 measurements arecollected to fill out the matrix. The dots filling in the diagonal ofthe matrix, which are substituted in place of the diagonal values ofR11, R22, R33, and so on, signify that these particular values are notvalid because they do not involve measurements across a wire pair andare therefore ignored. The resistance measurement values are notsymmetrical between the upper triangular values and lower triangularvalues. For example, R12 is not necessarily the same as R21 because theresistance measurement test which provides a d.c. test current has itspolarity reversed between these measurements. If there is a diodejunction in the current path, the junction will be forward biased in onedirection and reverse biased in the other, resulting in a largedifference in apparent resistance between the two polarities. Thisphenomenon is used to advantage when a cable identifier is connected tothe far-end of the cable for the purpose of verifying the connections ofthe LAN cable system.

Referring back to FIG. 6A, the microcontroller 160 moves on to decisionblock 230. Each resistance measurement value is compared to apredetermined figure of merit for short-circuit errors with a valuechosen to separate true short-circuit errors from otherwise normalresistance values, such as those presented by a cable identifier, with aminimum probability of false indications. If any resistance measurementvalue is less than the figure of merit for short-circuit errors, themicrocontroller 160 branches to display block 240 in which the detailsof the short-circuit error found are provided to the user through thedisplay 170.

The microcontroller 160 moves on to decision block 250 to detect thepresence of the cable identifier at the far-end of the LAN cable system.If no cable identifier is detected, the resistance measurement test islimited to detecting short-circuit errors between wire pairs that shouldotherwise all be open circuits.

If coupled to the LAN cable system, the cable identifier provides theinstrument 100 with predetermined resistance values in series withsemiconductor diodes in each of the expected wire pairs of 1-2, 3-6,4-5, and 7-8 in the commercial embodiment. The predetermined resistancevalue provides unique identification of the cable identifier number,with up to eight different cable identifiers available in the commercialembodiment. The microcontroller 160 compares each of the resistancemeasurement values to a series of figures of merit consisting of pairsof upper limit constants and a lower limit constants. The set ofconstants is based on the expected range of values of the provided cableidentifiers and are carefully chosen so as to provide a highly certaindetermination of the presence of the cable identifier as well as itsidentifier number in spite of the contributions of the resistance of theintervening LAN cable system for cable lengths up to 1,000 feet. Forexample, if a measured resistance value is greater than lower limitvalue and less than upper limit value for cable identifier 1, themicrocontroller 160 concludes that cable identifier 1 is present and themicrocontroller 160 then branches to a series of steps based on thepresence of the cable identifier, starting with display block 260.

In display block 260, the number of the cable identifier is displayed tothe user using the display 170. Using the previous example, the presenceof cable identifier 1 would thereby be indicated to the user. Themicrocontroller moves on to decision block 270 to check for errors inthe wire map of the LAN cable system. A wire map is a completecharacterization of the connections in the LAN cable system which iscoupled between the instrument 100 and the cable identifier, utilizingthe expected current directions dictated by the diode pairs in the cableidentifier to check for all possible wiring errors and generateindicator values responsive to the directions. The wire map is checkedby comparing the indicator values for each of the expected cable pairsto a corresponding expected indicator value. For example, the pair 1-2,is checked by comparing R12 and R21 to respective, pre-determined upperand lower limit values. Since the pair 1-2 has a diode and predeterminedresistor value in series as provided by cable identifier, the values ofR12 and R21 determine whether the pair is properly connected with thecorrect polarity. If wire map errors are detected, the microcontroller160 branches to display block 280 to display the errors in the wire mapto the user via display 170.

Referring now to FIG. 8, a symbolic matrix 510 provides storage for theresults of the decision process performed in blocks 230-280 operating onthe resistance matrix 500. The resistance measurement values ofresistance matrix 500 are first examined for short circuit wiring errors50d between any two wires. Any measured resistance values that are belowa predetermined figure of merit will be deemed to be a short-circuit andassigned an indicator value "0" in the respective position in thesymbolic matrix 510. Next, the presence of diodes and open circuits isexamined by comparing the measured resistance values of resistancematrix 500 to pre-determined figures of merit which are a series ofpairs upper and lower resistance values which are based on expectedresistances provided by one of a corresponding set of cable identifiers.If the resistance values are between the upper and lower limits, therespective position in the symbolic matrix 510 is assigned an indicatorvalue "+" and the transverse matrix position is assigned an indicatorvalue "-". If the measured value is above the highest upper resistancevalue expected for a cable identifier, the respective position in thesymbolic matrix 510 is deemed to be an open circuit and assigned anindicator value "*". As shown in symbolic matrix 510 as an example, wirepairs 1-2, 3-6, 4-5 and 7-8 were found to have diodes in theirrespective paths with the polarities assigned to the respectivedirections through each wire pair and no short-circuit wiring errors 50dwere found.

From this visual perspective of the collected indicator values providedby symbolic matrix 510, identifying errors in the wire map becomes amatter of comparison between the expected pattern developed for a knowngood LAN cable and the pattern identified. A LAN cable system with nocable identifier at the far-end should appear entirely as a pattern ofopen circuit "*" indicator values. A LAN cable system with a cableidentifier connected at the far.-end should appear primarily as apattern of open-circuits but with "+" and "-" indicator values in theappropriate locations in the symbolic matrix. As different cable typesbecome available, the figures of merit for comparing measurement valuesfor detecting short-circuit errors, diode paths, and open-circuit errorsmay be modified and a new overall decision pattern of indicator valuescustomized to match the new cable type by those having ordinary skill inthe art.

Referring now to FIG. 9, the data structure in which the capacitancemeasurement values are stored is a capacitance matrix 520. A total of 28measurements are collected to fill out the matrix. The dots that arealong the diagonal of the matrix, which are substituted in place of thediagonal values of C11, C22, C33, etc. signify that these particularvalues are not valid because they do not involve measurements across awire pair and are therefore ignored. The transverse matrix values of thecapacitance matrix 520, such as C12 and C21, are equal because thecapacitance measurement is an a.c. (alternating current) typemeasurement that does not have a single polarity.

Referring now to FIG. 6B, the measurement of capacitance values betweenall pairs of wires is conducted in measurement block 290. Themicrocontroller 160 then moves to block 300 to look for near-endopen-circuit errors. Open wires at the near-end connection in the LANcable system result in capacitance measurements that are significantlyless than those of a properly connected wire pair. The maximumcapacitance value is measured first and all the other capacitance valuesare compared with a predetermined fraction of that maximum value. If anyof the other capacitance values are less than this predeterminedfraction of the maximum value, they are deemed to be open circuits andgenerate a corresponding error. The predetermined fraction is a figureof merit that depends on the cable parameters and the maximum length ofcable from the near-end in which an open-circuit error can be resolved.For example, in the commercial embodiment, capacitances less than 10% ofthe maximum value are deemed to be open-circuit wiring errors. If anopen circuit is detected, the microcontroller 160 branches to thedisplay block 310 to display the open circuit to the operator viadisplay 170.

The microcontroller 160 then moves to decision block 340 to determine ifthe valid wire pairs found correspond to the expected pairings. In thiscase, wires 1-2, 3-6, 4-5, and 7-8 should be the identified valid pairs.If there is a problem detected in obtaining four valid pairs, themicrocontroller 160 branches to display block 350 to display an errormessage to the operator via the display 170.

The microcontroller 160 then moves to decision block 320 to check forfour valid wire pairs. Twisted wire pairs have higher mutual capacitancethan wires in that same cable that are not twisted together. The fourhighest capacitance values among the measured data are selected andtheir validity as matched pairs is checked by comparing their valueswith the values of the other non-paired capacitance measurements. To bea valid matched pair, each ratio must exceed a predetermined figure ofmerit for the minimum ratio of paired capacitance to unpairedcapacitance for all measurements. In the commercial embodiment of thepresent invention, a standard data grade LAN cable must have a minimumexpected ratio of 2.0. Other figures of merit may readily be derivedwithout undue experimentation by one skilled in the art. Ratios lessthan the figure of merit of 2.0 indicate any of a number of problemsthat might include a cable that is not adequate for data communicationssuch as voice-grade telephone cable. If there is a problem detected inobtaining four valid pairs, the microcontroller 160 branches to displayblock 330 to display an error message to the operator via the display170. Some types of LAN cables are twisted together in such a way thateight valid pairs are generated rather than four. In this specialsituation, no error is generated in decision block 320 for having toomany matched pairs.

Referring now to FIG. 10, a symbolic matrix 530 provides storage for theresults of the decision process performed in blocks 290-350 operating onthe capacitance matrix 520. The capacitance measurement values ofcapacitance matrix 520 are first examined for near-end open-circuitwiring errors 50c between any two wires. Any measured capacitance valuesthat are below a predetermined figure of merit will be deemed to be anopen-circuit wiring error and assigned an indicator value of "*" whichis stored in the respective position in the symbolic matrix 530. Thehigher value capacitances associated with twisted-wire pairs aredetermined as against the capacitances of wires not twisted together.The twisted-wire pairs are assigned an indicator value "H" in thesymbolic matrix and the other wires are assigned an indicator value "L".As shown as an example test result in the symbolic matrix 530, wirepairs 1-2, 3-6, 4-5 and 7-8 were found to be twisted-wire pairs and noopen-circuit errors were identified.

From this visual perspective provided by symbolic matrix 530,identifying errors in the pairing of wires becomes a matter ofcomparison between the expected pattern of indicator values developedfor a known good LAN cable and the pattern of indicator values found.The indicator value pattern is unaffected by the presence or absence ofthe cable identifier connected at the far-end of the cable. As differentcable types become available, the figures of merit for detectingopen-circuit errors and twisted-wire pairings may be modified andoverall decision pattern customized to match the new cable type by thosehaving ordinary skill in the art.

Referring now to FIG. 11, the microcontroller 160 calculates acapacitive difference matrix 540 based on the data from the capacitancematrix 520. Each entry in the capacitive difference matrix is thedifference between the capacitance measured from each wire of therespective pair to a third wire. The NEXT isolation of wire pair 1-2 isevaluated in the first row of the difference capacitance matrix 540. Thefirst two entries, which would be C11-C21 and C12-C22 respectively, aremeaningless values because they each involve the diagonal values C11 andC22 which are meaningless values from the original capacitance matrix520 and are thus substituted with dots and subsequently ignored. Thethird entry in the top row of difference capacitance matrix 540 isC13-C23 which is the figure of merit for the isolation between wire pair1-2 and wire 3. Similarly, the fourth entry in the top row of differencecapacitance matrix 540 is C14-C24 which is the figure of merit for theisolation between pair 1-2 and wire 4. The other entries in thecapacitance difference matrix 540 are developed in a like manner for theassigned wire pairs 3-6, 4-5, and 7-8 which make up the second, third,and fourth rows respectively. The values of the capacitive differencematrix 540 are used to verify near-end cross talk (NEXT) isolationbetween the identified wire pairs and all the other wires in the LANcable by comparing each value to a figure of merit correlated to athreshold acceptable level of NEXT isolation and generating an errorsignal in response to values that exceed the figure of merit.

Referring now to FIG. 12, the physical significance of capacitancedifference for each entry is illustrated in a cross-section view of theLAN cable 10 containing the wires 40. As shown, the wire pair 1-2 isevaluated for the difference in capacitance measured to a third wire 3which is expressed as C13-C23 corresponding to the appropriate entry inthe difference capacitance matrix. Similar, the difference calculationbetween pair 1-2 and wire 4 is illustrated as shown with thecapacitances connected with dashed lines. The other entries in thecapacitance difference matrix 540 are calculated in a like manner. Thevalue of the capacitive difference correlates to NEXT values measuredthrough prior art analog techniques which measure the signal isolationdirectly by inducing a voltage in the wire pair and measuring the ratioof induced signal to applied signal. If the capacitive difference isrelatively low, any voltage induced across C12 in the wire pair 1-2 bywire 3 will be equal and therefore no differential voltage will becapacitively induced across the wire pair from voltages present in wire3. However, if the difference capacitance is significantly large, theNEXT isolation of the wire pair 1-2 to wire 3 deterioratesproportionately. Accordingly, a figure of merit for maximum differentialcapacitance was derived that correlates to an acceptable level of NEXTisolation which in turn is determined by industry specifications.

Referring now to FIG. 6C, the microcontroller 160 moves to decisionblock 360 to calculate the capacitance difference matrix 540 asdiscussed above. The microcontroller 160 then moves on to decision block370 to perform the NEXT isolation check in which each entry in thecapacitance difference matrix 540 is compared with the figure of meritfor NEXT isolation and, if any entry exceeds that figure of merit, themicrocontroller 160 branches to a display block 380 to display thedetected NEXT problem to the operator via the display 170.

The microcontroller 160 moves to decision block 390 to determine whetherany prior errors have been encountered during the entire test phase. Ifno errors have been detected, microcontroller 160 branches to displayblock 400 to alert the operator that no errors have been found. The testsequence as illustrated in FIG. 6A, 6B, and 6C then terminates in block410.

It has been shown herein a method for testing LAN cables. A series ofresistance measurements is made between all possible wire pairs andstored in a resistance matrix where decisions are made regardingshort-circuit errors, and, if the cable identifier is present, diodepolarities and cable identifier numbers by comparing the resistancemeasurement values to a predetermined set of figures of merit. Theindividual decisions are stored symbolically in a symbolic matrix asindicator values and the pattern of indicator values is compared to aknown good pattern. A series of capacitance measurements is then madebetween all possible wire pairs and stored in a capacitance matrix wheredecisions are made regarding open-circuit errors and wire pairings bycomparing the capacitance measurement values to a predetermined set offigures of merit. The individual decisions are stored symbolically inanother symbolic matrix and the pattern of symbols is compared to aknown good pattern. Finally, the near-end cross talk isolation betweenidentified cable pairs is evaluated by calculating a differencecapacitance from the values stored in the capacitance matrix and theresulting difference capacitance values are compared to a predeterminedfigure of merit correlated to an acceptable threshold level of NEXTisolation.

It will be obvious to those having ordinary skill in the art that manychanges may be made in the details of the above described preferredembodiments of the invention without departing from the spirit of theinvention in its broader aspects. For example, different types ofswitches may be substituted for the switch matrices 130 and 140.Different user interface technologies may be substituted for display 170and user control 180. The methods of evaluating the resistance andcapacitance measurements may be readily tailored and the sequence ofprogrammed tests readily altered to adapt to new types of LAN cableconfigurations and systems as they arise. Therefore, the scope of thepresent invention should be determined by the following claims.

What we claim as my invention is:
 1. A LAN cable test instrument fortesting a LAN cable having a near-end and a far-end, comprising:(a)input means for coupling to a plurality of wires from said LAN cable atsaid near-end, said plurality of wires forming twisted-wire pairs withinsaid LAN cable; (b) first and second switch means coupled to said inputmeans, said first switch means selectively coupling a first wire of saidplurality of wires to a first output and said second switch meansselectively coupling a second wire of said plurality of wires to asecond output; (c) measurement means with a first input coupled to saidfirst output and a second input coupled to said second output, saidmeasurement means measuring the resistance and the capacitance betweensaid first and second wires to produce resistance and capacitancemeasurement values; (d) memory means coupled to said measurement meansfor storing said resistance and capacitance measurement values; and (e)a microprocessor coupled to said said memory means wherein saidmicroprocessor operates on said resistance and capacitance measurementvalues to determine said twisted-wire pairs in said LAN cable.
 2. A LANcable test instrument according to claim 1 wherein said memory meanscontains said resistance and capacitance measurement values for eachpair of said plurality of wires formed by said first and second wires.3. A LAN cable test instrument according to claim 2 wherein saidmicroprocessor calculates a difference capacitance matrix from saidcapacitance measurement values in said memory means to evaluate near-endcross-talk isolation between each of said twisted-wire pairs.
 4. A LANcable test instrument according to claim 1 wherein said microprocessorcompares said resistance and capacitance measurement values against afirst predetermined value to detect a short circuit between said firstand second wires.
 5. A LAN cable test instrument according to claim 1wherein said microprocessor compares said resistance and capacitancemeasurement values against a second predetermined value to detect anopen circuit between said first and second wires.
 6. A LAN cable testinstrument according to claim 1 further comprising a display coupled tosaid microprocessor for displaying said twisted-wire pairs of said LANcable.
 7. A LAN cable test instrument according to claim 1 furthercomprising a cable identifier having an identifier number, said cableidentifier coupled to said far-end of said LAN cable to provide apredetermined resistance value associated with said identifier numberacross said first and second wires.
 8. A LAN cable test instrumentaccording to claim 7 wherein said microprocessor compares saidpredetermined resistance value against a series of figures of merit todetermine said identifier number.